Merge branch 'stable-3.4'

* stable-3.4:
  Update documentation about mandatory properties
  Send/receive Event object instead of EventMessage
  Use EventGsonProvider from Gerrit core
  Bump events-broker version to v3.4.0.1
  Add dummy settings for aws credentials
  Remove unmaintained attempt to Dockerize the whole multi-site setup
  Implements async send interface
  Change source instance id type from UUID to String

Change-Id: I02801041a6edd60e5c63cbda294117c09ff56a19